A new class of dark matter-free dwarf galaxies?

I. Clues from FCC 224, NGC 1052-DF2, and NGC 1052-DF4

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
² Centre for Astrophysics and Supercomputing, Swinburne University, John Street, Hawthorn, VIC 3122, Australia
³ Department of Physics and Astronomy, San José State University, One Washington Square, San Jose, CA 95192, USA
⁴ Department of Astronomy & Astrophysics, University of California Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA
⁵ Instituto Astrofisica de Canarias, Av. Via Lactea s/n, E38205 La Laguna, Spain
⁶ Departamento de Astrofisica, Universidad de La Laguna, E-38200 La Laguna, Tenerife, Spain
⁷ Dragonfly Focused Research Organization, 150 Washington Avenue, Santa Fe, NM 87501, USA
⁸ University of California Observatories, 1156 High Street, Santa Cruz CA 95064, USA
⁹ Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 München, Germany

^⋆ Corresponding author; marialuisa.buzzo@eso.org, lgomesbuzzo@swin.edu.au

Received: 19 December 2024
Accepted: 6 February 2025

Abstract

Context. The discovery of quiescent, dark matter (DM)-deficient ultra-diffuse galaxies (UDGs) with overluminous globular clusters (GCs) has challenged galaxy formation models within the Lambda cold dark matter (ΛCDM) cosmological paradigm. Previously, such galaxies were only identified in the NGC 1052 group, raising the possibility that they are the result of unique, group-specific processes, and limiting their broader significance. The recent identification of FCC 224, a putative DM-deficient UDG on the outskirts of the Fornax Cluster, suggests that such galaxies are not confined to the NGC 1052 group but rather represent a broader phenomenon.

Aims. We aim to investigate the DM content of FCC 224 and to explore its similarities to the DM-free dwarfs in the NGC 1052 group, DF2 and DF4, to determine whether or not it belongs to the same class of DM-deficient UDGs.

Methods. We use high-resolution Keck Cosmic Web Imager (KCWI) spectroscopy to study the kinematics, stellar populations, and GC system of FCC 224, enabling direct comparisons with DF2 and DF4.

Results. We find that FCC 224 is also DM-deficient and exhibits a distinct set of traits shared with DF2 and DF4, including slow and prolate rotation, quiescence in low-density environments, coeval formation of stars and GCs, flat stellar population gradients, a top-heavy GC luminosity function, and monochromatic GCs.

Conclusions. These shared characteristics signal the existence of a previously unrecognised class of DM-deficient dwarf galaxies. This diagnostic framework provides a means of identifying additional examples and raises new questions for galaxy formation models within ΛCDM cosmology.